Conventionally, photonic infrared (IR) detectors employ low band gap materials such as InGaAs, InSb, or HgCdTe. However, these materials include elements that are rare, expensive, or toxic. Past research indicates that crystalline Si (c-Si), which is a much cheaper and more abundant element, could be used for IR detection when metal electrodes are properly nanostructured.
In this type of photodetection system, the IR light with energies below the c-Si band gap is strongly absorbed by the metal structures, rather than by c-Si. The photoexcited electrons in the metal can then be injected into the conduction band of c-Si before being thermalized, and electric current can be generated. These non-thermalized electrons, called hot electrons, enable the detection of IR light with energies below the c-Si band gap.
For efficient transport of electrons in the metal before thermalization, the metal layer should be as thin as approximately the electron mean free path. Accordingly, the metal layer thickness should be only a few tens of nanometers. To induce strong optical absorption in such a thin metal layer, surface plasmon polaritons (SPPs) can be excited at the metal surface. Previous studies on hot electron photodetection utilized small-scale metamaterials or deep trench resonators to have strong resonant absorption of SPPs in thin metal films on c-Si at the desired frequencies. However, these structures needed to be fabricated with high precision because the metal structure sizes determine resonances.
Accordingly, in many cases, expensive techniques such as electron beam lithography have been commonly used to fabricate the structures. However, for mass production, it is important to obtain metallic structures that do not require expensive techniques and tolerate practical fabrication errors.